The present invention relates to the automatic production of filter bags for infusion products such as tea, chamomile and similar herbs, and relates in particular to a very advantageous sealing apparatus for use in machines for making the filter bags.
In the automatic packaging of products for infusion, prior art teaches the use of machines which continuously form filter bags by folding and sealing lengthwise a web of heat-sealable filter paper to make a flattened tube positioned horizontally and then sealing this tube crossways at defined intervals in such a way as to enclose individual charges of infusion product between two successive transverse heat seals. The flattened tube is then cut crossways into lengths, to which a thread and a pickup tag are applied in order to complete the filter bags.
As regards the sealing operations, which form the specific subject-matter of the present invention, prior patent IT 1207628 discloses a sealing apparatus comprising two blocks conformed so as to present pairs of stepped overlapping edges. The upper edge of each pair has a plurality of bores through which hot air from a heater is passed. The lower edges, on the other hand, have a lamellar structure. The two pairs of edges operate in mutual juxtaposition, on the outside of the tube of filter paper, combining to channel a stream of hot air against the edges of the tube, the edges of the tube being placed side by side in close contact with each other to form the generatrix of the tube and moving between the pairs of edges of the sealing apparatus. The hot air directed by the bores thermally and locally activates a layer of glue on the inside surface of the tube of heat-sealable filter paper. As the tube of filter paper moves along its feed path, it passes between two successive pairs of pressure rollers which make the longitudinal and transverse seals on the tube in such a way as to create the pouches containing the individual charges of infusion product.
In this type of sealing apparatus, the location of the heating blocks and the emission of hot air from the outside of the filter paper tube being formed result in considerable energy loss and low efficiency of the apparatus. Indeed, although the hot air can be emitted from the bores at quite a high speed, the horizontal distance that it has to travel before reaching the vertical edges of the filter paper to be sealed, and the fact that the blocks incorporating the bores are positioned outside the filter paper tube inevitably lead to a part of the stream of hot air being lost by natural convection without producing any useful effect on the working area.
Another factor that negatively influences energy efficiency is that the hot air strikes the face of the filter paper opposite the face with the glue to be reactivated on it, the reactivation thus being the result of the conduction of heat through the filter paper. Although the filter paper is very thin and by nature microporous, this mode of operation nevertheless further increases the overall inefficiency of the apparatus.
Further disadvantages of the apparatus described above lie in its complex and rather cumbersome structure.
The main aim of the present invention is to overcome the above mentioned disadvantages by providing an apparatus designed in such a way as to direct the stream of hot air entirely on the edges to be sealed, avoiding conditions that enable the hot air to bypass the sealing area, and in such a way that the stream of air is directed straight at the layer of glue to be reactivated.
Other aims of the invention are to provide an apparatus that: has a simple structure; is relatively inexpensive to make; and occupies much less space than prior art devices designed for the same purpose.